1. Field of the Invention
This invention relates to systems for infusion monitoring and, more particularly, to an apparatus and method for preventing bubble formation in a drip chamber during filling or fast flushing.
2. Background Art
Hemodynamic monitoring of surgical and other seriously ill patients is a commonly accepted practice, vital to the medical management of the patient. In order to sense the desired pressure for registration or recording, a blood vessel in the patient must be cannulated. That is, a conduit filled with relatively incompressible liquids must be established from the pressure site within the patient out to an appropriate instrumentation system.
Although the conduit, or pressure-monitoring line, may be filled in its entirety with a liquid, such as sterile, isotonic saline solution containing a diluted anticoagulant agent, the tip of the cannula must interface with the patient's blood. Due to diffusion, and in arterial pressure procedures, affected by pulsations, the blood may coagulate, forming a clot that is hazardous to the patient and introducing serious errors in the pressure measurement. The coagulation process is aggravated by even the slightest leak in the liquid-filled system.
Since the cannula must often remain in place for an extended period in an unimpaired, clot-free condition, considerable effort has been expended to achieve this goal. A known solution is to incorporate a capillary tube connected to a pressurized source of liquid, allowing a slow, continuous flow through the cannula into the patient. A volume of two to four cubic centimeters (cc) per hour has been found effective in the case of an adult, without interfering with the ongoing pressure monitoring.
One variation of the system described above affords an optional rapid-flow mode from the pressurized solution source for initial filling and debubbling of the system and later flushing of the line after a blood sample has been taken. Typical systems in common use incorporate a drip chamber located near the pressurized liquid source, in series with the administration set line. The drip chamber has proved to be a primary source of air bubble formation.
This chamber is kept only partially full, so that liquid entering it forms drops from a tube, thereby establishing a visible indication of flow volume. Due to potential differences in the flow devices mentioned in the preceding paragraph, the rate may deviate from the specified range of two to four cc per hour. It is common practice to size the input tube so that each drop per minute into the chamber represents a flow volume of one cc per hour. By observation of the drip chamber for one minute, the flow rate can be determined.
Development of this desirable flow indicator has resulted in the use of a very small diameter drip tube. When the fast-flush mode of the flow device is actuated, the small diameter causes the liquid to form a high-speed jet. A Venturi effect results, causing air to entrain in the fluid stream, which creates great turbulence and aeration in the chamber and frothing of the liquid. A train of air bubbles will be drawn into the administration line and then into the flow device and on into the patient. Such bubbles are a life-threatening hazard to the patient and produce intolerable distortion in the pressure measurements.
One proposed solution to the problem of bubble formation is the installation of a filter at the lower region of the drip chamber. While the filter tends to intercept and screen out larger bubbles, microscopic bubbles will penetrate even very fine filtering at high flow rates and under severely turbulent conditions. This phenomenon may be observed when the otherwise clear, transparent solution appears somewhat gray in color as it passes through the filter. Later, colescence of these fine bubbles downstream in the administration line results in visible bubbles that constitute a hazard.
The present invention is directed to overcoming one or more of the problems enumerated above.